The present invention is in the field of regulating the average power applied to varying loads; particularly, this invention is applicable to oil field treater equipment.
An example of the type of equipment in which the present invention finds its primary, though not exclusive, application is Prestridge-Wallace U.S. Pat. No. 3,772,180. The Prestridge-Wallace apparatus is an oil treater that reduces the amount of water contained in oil. It applies an a.c. voltage across an interface in the vicinity of an oil-water emulsion. The amount of power drawn by this load can swing to rather high levels when the moisture content of the emulsion is high. If the average power applied to the load remains high, damage can be done to the treater equipment. Accordingly, it is necessary to regulate the power applied to the load.
Another patent issued to Prestridge-Wallace, U.S. Pat. No. 3,939,395, discloses a circuit for regulating the average power applied to the load. Basically, the apparatus in the U.S. Pat. No. 3,939,395 senses the a.c. current flowing through the load, rectifies and filters the representation produced by the sensing, and uses the result as an indication of whether or not to remove the power from the load. As long as this resultant filtered current overload signal remains above a predetermined level, the power remains removed from the load. When the filtered signal falls below the predetermined level, the apparatus reconnects the a.c. source at a predetermined relationship to the zero crossing of the voltage with the objective of avoiding high voltage transients in the transformer. Because the filtered signal is at a high value immediately after a large current has flowed through the load, the power is disconnected for some period of time until the voltage in the filter circuit has decayed below the predetermined level. If the current is relatively high, it takes a relatively long time for the voltage in the filter circuit to decay, thereby decreasing the duty cycle of the application of power to the load. If the current level is not quite so high, the duty cycle will be higher. If the current is low enough to allow continuous application of the source voltage without exceeding the average power, the voltage in the filter circuit will not be high enough to cause the power to be disconnected from the load, and the duty cycle is 100 percent. Thus, the average power level remains below that which will damage the treater apparatus.
While the apparatus of U.S. Pat. Nos. Prestridge-Wallace 3,939,395 and Bernstein 4,122,382 does act to protect the treater to some extent, it is now apparent that their systems contain certain deficiencies which require modification.
Neither the Prestridge-Wallace nor the Bernstein system accounted for the flux relationships in their transformers as their control systems connected and disconnected their line supplies to their transformers. Both systems concentrated on connecting and disconnecting as the a.c. line supply voltage goes through zero. Although these operations were in the right direction to avoid the threatening transients, the rationale of their systems did not properly account for the variations of residual flux in their transformers. The fact is that a control system cannot be made to operate the electronic switch in the line supply as if it were a resistive element whose characteristics are unrelated to the inductive characteristics of the load. If zero voltage switching is the function of the controller, reconnection of the voltage to the transformer will not occur at the proper time to match the residual flux of the transformer.
It is now clear that the a.c. voltage supply to inductive load must be reconnected in a lag relationship to the zero voltage crossing of the supply to insure that the flux demand is substantially equal to the residual flux remaining in the core of the transformer from the previous disconnection.